This invention pertains to wireless receivers. Without limitation, the background is described in connection with FM receivers. FM is popular in many developed countries and is growing in popularity in a number of developing countries for all kinds of receiver and devices including them.
With the growing popularity of FM transmission in a number of developing countries, as well as the developed countries, low-cost integrated FM receivers have become important to integrate into mobile handsets like cell phones and Internet devices as well as FM-supporting integrated circuits of various types for those and other products.
In FM (frequency modulation) receivers, audio sensitivity is a key parameter that determines the weakest received signal that can be demodulated with acceptable audio quality. Moreover, audio sensitivity is a problematic and important parameter for FM receivers as it can be perceived by the user. Improving the audio sensitivity is appealing both to manufacturers and to the user public because it enhances the range or distance a receiver can be located away from a given transmitter, and improves reception in various reception scenarios.
Finding ways to make audio sensitivity higher is a continuing challenge to the art. Moreover, consideration of ways to increase audio sensitivity encounters problems of increased current consumption and/or degradation of other performance parameters. In the mobile segment, any incremental current consumption to achieve an audio sensitivity improvement needs to be small or negligible, as the time between battery recharges is another key concern in mobile devices.
The audio sensitivity in FM receivers is suitably defined as the minimum signal strength at the RF (radio frequency) input of the FM receiver that results in a specified demodulated audio signal to noise ratio (SNR). The audio sensitivity performance for FM is measured for a specified audio frequency deviation in vibrations per second or kilohertz (KHz).
In the United States and Europe, FM broadcast stations use a bandwidth of 200 KHz assigned to them at different frequencies or positions within the 87.5 MHz to 108 MHz. In Japan the FM band or available frequency spectrum is a 76 MHz to 90 MHz band. There, an FM channel can be centered at multiples of 50 KHz, with a frequency spacing of at least 200 KHz between any two valid stations. The FM center frequency can be centered at multiples of 50 KHz in some parts of the world and at multiples of 100 KHz in other parts of the world.
In FIG. 3, a MPX (multiplex) signal is shown in a spectrum diagram of amplitude versus frequency carrying stereo left L and right R audio signals for ultimate listening, but in a combined form. The FM MPX signal has a mono (L+R) component at audio frequency, a pilot at 19 KHz, a stereo difference component—left minus right (L−R)—translated up around 38 KHz, and an RDS (radio data system) signal. (An FM receiver can use a radio data system (RDS) circuit or a radio broadcast data system (RBDS) circuit for processing specific data, e.g., station identification, song title, time, program identification, and name of artists, received from an FM broadcast station.)
The audio signals, pilot signal and RDS signal components can have different spectrum amplitudes vertically, as graphed across the horizontal frequency axis of FIG. 3, and they combine together in a complicated but straightforward way to form the instantaneous modulating MPX signal in the time domain. The FM carrier of a given FM broadcast station is frequency modulated (FM) by that time domain MPX signal to generate its FM broadcast signal. Frequency deviation on the outside or leftmost vertical axis of FIG. 3 is the amount of frequency variation of the FM broadcast signal at RF that a given combined time domain varying instantaneous voltage of a MPX modulating signal causes, as indicated by the encompassing dotted line in FIG. 3. The audio signals, etc. of the MPX signal are frequency modulated (FM) onto the RF carrier at the FM transmitter. The frequency of the RF carrier with no modulation corresponds to the nominal frequency location of the FM signal in the FM radio band.
Thus the audio signals combined as L+R and L−R are frequency modulated (FM) onto an RF carrier, and the occupied RF channel bandwidth is approximately indicated by the outside vertical axis in FIG. 3. That occupied RF channel bandwidth, also called the audio deviation here, depends on the instantaneous amplitude of the time-domain MPX signal. Hence the audio deviation can vary significantly. A loud audio signal causes a large audio deviation, meaning that the FM carrier at RF is modulated to vary in frequency over a wider frequency range at RF than it would with a soft audio signal. Notice that the RF amplitude of the transmitted FM signal from the FM broadcast station does not vary in a significant way with the audio loudness. The RF amplitude, as well as RF SNR (signal-to-noise ratio), do depend on the power of the FM broadcast station, the distance to the receiver, terrain and obstacles to propagation. According to the regulations of most countries, the FM broadcast signal can have an occupied bandwidth of up to 75 KHz, and the actual occupied bandwidth is determined by the combined intensities or amplitudes across the spectrum of FIG. 3.
The audio sensitivity of an FM receiver in one possible approach might be improved by reducing the noise figure of the analog RF front-end of the FM receiver. However, noise figure reduction comes at the cost of increased current consumption that leads to shorter battery life in mobile devices, as well as potentially increased integrated circuit chip area that means the chip is more costly to make. For instance, reducing the noise figure of the FM receiver from 5 dB to 3 dB might increase the electric current consumption of an analog front-end electronic receiving circuit by milliamperes. This means more drain on the battery and shorter battery life and more recharges called for, which can inconvenience the user.
Remarkable new ways and departures to increase the audio sensitivity would thus be very desirable in this technological art—while substantially controlling current consumption and preserving concomitant battery life, and maintaining other performances undegraded like Total Harmonic Distortion and RDS sensitivity, and keeping the integrated circuit chip area economical.